Mass Diffusivity Input: const-dillute-appx
 

During a "Species Transport" simulation, when I wanted to define constant mass diffusivity for the mixture, surprisingly, I could not find the "constant-dilute-appx" option in the drop down list. I am using Fluent 16.0 version. I have attached the screenshot herewith.

For my case, it is important to give input for Mass Diffusivity and hence, I cannot select "kinetic theory". I would like to ask about the possible solution for this problem. Is there any problem with settings?

Thanking you.